Janino is a super-small, super-fast Java compiler.
Janino can not only compile a set of source files to a set of class files like JAVAC,
but also
- compile a
Java expression - a
block - a
class body - one
.java file - a
set of .java filesin memory,
load the bytecode and execute it directly in the same JVM.
Let's say we build an e-commerce system, which computes the shipping cost for the items that the user put into his/her shopping cart.
Because we don't know the merchant's shipping cost model at implementation time,
we could implement a set of shipping cost models that come to mind (flat charge, by weight, by number of items, ...) and select one of those at run-time.
In practice, we will most certainly find that the shipping cost models we implemented will rarely match what the merchant wants,
so we must add custom models, which are merchant-specific.
If the merchant's model changes later, we must change our code, re-compile and re-distribute our software.
Because this is so unflexible, the shipping cost expression should be specified at run-time, not at compile-time.
This implies that the expression must be scanned, parsed and evaluated at run-time, which is why we need an expression evaluator.
Important-
A simple expression evaluator would parse an expression and create a "syntax tree".
The expression "a + b * c", for example,
this would compile into a "Sum" object who's first operand is parameter "a" and who's second operand is a "Product" object who's operands are parameters "b" and "c".
Such a syntax tree can evaluated relatively quickly.
However, the run-time performance is about 100 times slower than that of "native" Java code executed directly by the JVM.
This limits the use of such an expression evaluator to simple applications.
Also, we may want not only do simple arithmetics like "a + b * c % d",
but take the concept further and have a real "scripting" language which adds flexibility to our application.
Since we know the Java programming language already,
we may want to have a syntax that is similar to that of the Java programming language.
Compiling Java programs with ORACLE's JDK is a relatively resource-intensive process (disk access, CPU time, ...).
This is where Janino comes into play... a light-weight, embedded Java compiler,
that compiles simple programs in memory into JVM bytecode which executes within the JVM of the running program.
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